Coating apparatus and method

ABSTRACT

The invention is directed toward a method and apparatus which can be used to allow the sputter deposition of material onto at least one article to form a coating on the same. The new form of magnetron described herein allows an increase in sputter deposition rates to be achieved at higher powers and without causing damage to the coating being created. This can be achieved by improved cooling and use of a relatively high magnetic field in the magnetron while at the same time increasing the power to the magnetron by increasing the current at a rate faster than the voltage.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is the US National Phase of PCT Application No.GB/2007/002600 filed 12 Jul. 2007 which claims priority to GBApplication No. 0613877.0 filed 13 Jul. 2006 and to GB Application No.0707801.7 filed 23 Apr. 2007, each of which are incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable

INCORPORATED-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

The invention to which this application relates is to apparatus for usein the application of a coating, typically by sputtering of materialfrom one or a series of targets provided as part of respectivemagnetrons.

The invention also relates to the provision of improved coatings whichare corrosion resistant and therefore allow the protection of articleswhich otherwise would be prone to corrosion in their normalenvironmental conditions.

The application of material onto articles to form a coating layer orlayers using the sputtering of material from targets located withmagnetrons is well known. There are several different forms of apparatuswhich can be used, one of which is referred to as a closed fieldunbalanced magnetron array with the magnetrons being provided in aspaced array within a coating chamber which is held in a vacuum. Thearticles to be coated are selectively placed in the coating chamber andmay be moved so as to allow the exposure of the articles to thesputtered material from the targets. The selective activation of thetargets and/or selective introduction of gases into the coating chamber,allow the particular form of layers and the formation of the layers tobe adjusted and hence the coating to be formed in a desired manner.

A known problem with magnetron sputtering apparatus is that thedeposition rates can be relatively slow in comparison to other coatingforming means. This has therefore tended to mean that while the qualityof the coatings which are applied are of a standard so as to make thesame attractive for relatively high cost articles, the coating of lowercost articles using the magnetron sputtering method and apparatus maynot always be commercially practical. This therefore means that articlesmay be coated by other deposition methods which have a higher depositionrate but frequently, the coatings are of inferior quality. One such typeof article is a plate for a fuel cell, typically for use in automobileor other vehicle power systems. The fuel cell plates are used in liquidwhich is corrosive and therefore the fuel cell plates need to beprotected from corrosion. Conventionally this is achieved by coating thefuel cell plates with an inert metal such as precious metal such as goldor platinum. These materials are expensive. It is also important thatthe coating which is applied is conductive in this particular use andindeed in many other uses there is a desire to be able to provide aconductive, corrosion resistant coating.

BRIEF SUMMARY OF THE INVENTION

One aim of the invention is to provide apparatus which allows for thesputter deposition of material to form a coating with an increaseddeposition rate and to do so whilst at least maintaining the quality ofthe coating which is achieved by conventional sputtering deposition. Asecond aim is to provide an alternative conducting, corrosion resistant,coating for articles and furthermore to provide a coating which hasimproved tribological properties.

In a first aspect of the invention there is provided a magnetron for thesputter deposition of material from at least one target of a materialmounted in association with the magnetron, said magnetron including amagnet array comprising a series of spaced magnets, means to allow theintroduction and passage of a cooling fluid, and a power supply andwherein when increasing the level of the power supply there is at leasta phase during which the current increases at a greater rate than thevoltage level.

In one embodiment during said phase the voltage level is substantiallyconstant.

Typically the said phase occurs after an initial phase at thecommencement of operation of the magnetron during which initial phaseboth voltage and current levels increase. Typically during the initialphase the current and voltage increase at a similar rate.

In one embodiment the magnetron includes an inlet located at or adjacentto a first extremity of the magnetron and an outlet located at oradjacent to a further extremity, in one example the opposing extremity,of the magnetron to thereby allow liquid which passes through themagnetron to pass across substantially all of the area of the rear faceof the target of the magnetron.

Preferably, the inlet and outlet are located such that the liquid passesacross all of the rear face of the target thereby providing an improvedcooling effect on all of the target and the magnet array.

Preferably, the outlet and inlet lie in line with a magnet or magnetsmounted at the periphery of the magnetron and, yet further, to the rearof the said magnet or magnets. This position ensures that the liquidflowing through the passage extends across the entire portion of therear surface of the target. This therefore prevents the build-up of anair pocket in the passage which is found to conventionally occurespecially when the outlet does not lie in line with the magnet butinstead lies in part of the passage which lies inwardly of the outermagnet array. The current invention will therefore prevent the commonoccurrence of air pockets at the top of the magnetron, when themagnetron is used in a vertical orientation.

In one embodiment, the liquid flows through the magnetron with aturbulent flow.

In one embodiment, the magnet array is located to the rear of the targetand the channel through which liquid flows between the magnet array andthe rear of the target is less than 5 mm in depth and preferably in theregion of 2-3 mm.

In one embodiment, the magnets which are used for the magnet array areof a corrosion resistant material, such as samarium cobalt magnets.

Typically, because the magnets are made of a corrosion resistantmaterial, the same can be exposed to the cooling liquid and hence thefront faces of the magnets are spaced in contact or close to contactwith the rear face of the target.

In one embodiment, the gap between the front face of the magnets and therear face of the target is in the region of 1-2 mm.

In one embodiment the power supplied to the magnetrons can be increasedto a greater level than would be possible in a conventional magnetronand still allow the generation of acceptable and, in some instances,improved coating quality. Conventionally, if the power supply to amagnetron is raised above a certain level, the quality of the coatingcan deteriorate rapidly. It is found that using a magnetron or indeed anumber of magnetrons in accordance with the invention, allows the powerto be increased with the voltage only rising by a relatively smallamount thereby allowing the quality of the coating to be maintained andin some instances improved while at the same time allowing theapplication of the coating to be increased in terms of speed ofdeposition and hence increased speed of throughput of articles.

In one embodiment, the magnetrons are mounted in apparatus in the formof an in-line coating system and typically, opposing pairs of depositionmagnetrons are located along a longitudinal axis with each of themagnetrons in the pair sputtering material towards each other with thearticles to be coated passing therebetween so as to allow the materialto be applied to both sides of the article simultaneously.

In one embodiment the facing magnetrons are of opposite polarity to forma closed field arrangement.

In one embodiment, the coating which is applied using the magnetrons inaccordance with the invention is a conductive, corrosion resistantcoating. In one embodiment the coating is a carbon coating in which thecarbon carbon bonds are mostly of the sp2 form.

In a further aspect of the invention there is provided a magnetron, saidmagnetron having a material target forming at least part of the frontsurface of the same, a supporting frame and to the rear and/or side ofthe target, a series of magnets formed as a magnet array, said magnetarray including a series of magnets provided around the periphery of thetarget, and at least one magnet located substantially centrally of thetarget and wherein, intermediate said first and second magnets, there isprovided at least one item of non-magnetic material, said material lyingto the rear of the target and provided to form at least part of achannel along which a cooling liquid passes.

In one embodiment the non-magnetic material is of plastic or aluminium.

In one embodiment, the said items of non-magnetic material which areused, embed or enclose items of magnetic material which are located toinfluence the sputter deposition of material from the targets of themagnetrons.

In order to improve the deposition rate of material from the target ofthe magnetron the power applied to the magnetrons is increased and thecooling effect which is required to prevent the magnetron overheating asa result of this increase in power is achieved by the improved coolingof the magnetron significantly. The redesign of the cooling channel toensure turbulent flow, which is more efficient than the conventionallaminar flow, and the positioning of the liquid inlet and outlet at theextreme respective ends of the magnetron so that there are no “dead”regions allow the cooling effect to be improved.

With the improved cooling the power which can be applied to themagnetrons can be increased greatly with respect to that which wasconventionally possible and indeed it has been found that the onlybarrier to increasing the power is the practicalities of obtaining asufficient large power supply.

This is in contrast to conventional magnetrons which have thecharacteristic that if both the current and voltage increase at aboutthe same rate then problems occur in that if the voltage on a target ofgraphite carbon material in a magnetron goes over a critical value (say−550V) then there is arcing at the surface of the graphite and solidparticles of carbon are produced which can deposit on the articles,hence producing defects and the particles can subsequently fall out ofthe coating leaving pores which is unsatisfactory for a corrosionresistant coating.

The increase in power which can be achieved and which is useful can bedetermined with respect to the particular coating material which is tobe applied from the target of the magnetron. For example, if the coatingwhich is to be applied is a graphitic coating the limitation to thepower which is used on the magnetron of the invention may be caused atthe moment by the breakdown of the bond between the carbon target andthe copper backing plate in the magnetron. If the bond is improved toresist this then the power used can be increased further. However withother target materials, the restriction may not apply, and the power maybe increased yet further, for example by 6 times the conventional powersupply. This may then cause other practical problems to be encounteredsuch as overheating of sealing means used with the magnetron.

In accordance with a further aspect of the invention there is provided amagnetron with a magnet array and wherein the arrangement of the magnetsis configured to increase the power applied to the magnetron such thatthe current increases while the voltage remains almost constant.

In this manner it is found that as the voltage increase is minimised sothe problems of arcing and poor quality coatings is avoided while at thesame time allowing for the increase in power and hence deposition rateincrease.

Typically the magnetron can receive and operate with increased power ofat least 3 times the conventional power level.

The improved cooling which is achieved allows the application of higherpower to the magnetron. The use of strong SmCo magnets close to the rearsurface of the target produces very large magnetic field strength acrossthe front surface of the target. These large magnetic fields influencethe voltage-current characteristics of the magnetron such that when thepower applied is increased large current increases are obtained alongwith relatively small voltage increases which is a very desirablecharacteristic for the application for coatings such as the conductive,corrosion resistant coating herein described. Conventionally with thistype of coating a voltage higher than about 550V causes arcing andparticles of carbon to be deposited as previously stated, so theimprovement in the coatings at higher deposition rates which is achievedin accordance with this invention is very significant. Thecharacteristics will also allow the deposition of a wide range ofmaterials at high rates without the limit of voltage level, which isimposed by many power supplies intended for magnetron sputtering, beingreached.

In a further aspect of the invention there is provided a magnetronhaving a target of material in association therewith and from whichmaterial is required to be deposited and wherein the magnetron, with atarget length of 380 mm and width of 175 mm, when provided with a targetof Cu, can be operated at power up to 30 kW without damage to themagnetron to provide a coating on a substrate with no droplet typedefects detected using SEM examination of the coating.

In one embodiment the cooling fluid provided in the magnetron is waterand the temperature of the same is 35 degrees Celsius.

In one embodiment when the power is 28 kW, and the substrates to becoated are held on a rotating carrier at 4 rpm at a distance of 150 mmfrom the target the deposition rate from the target 30 microns per hour.

It has been found that the current-voltage characteristics indicate thatmuch higher power could be used. The deposition rate of the magnetronrival that of arc sources but without the associated droplet formation.

In one embodiment a magnetron with an industrial size 665 cm² sputtertarget and with high power density (up to 40 W/cm²) was used.

Typically a relatively strong magnetic field is created at and adjacentto the sputter face of the target of the magnetron and the magnetron iscapable of sputter regimes at high power but low voltage.

In one embodiment the magnetron is operated with aluminium, titanium orgraphite material targets and mounted with in a deposition chamber inwhich the pressure can be adjusted to suit specific requirements.

In one embodiment a large increase in the current which can be appliedto the magnetron is achieved with little or no increase in voltage,indicating efficiency of electron confinement near the target andpossibly self-sputtering at higher power densities.

Typically the substrates to which the coating is applied are biased.

In a further aspect of the present invention there is provided apparatusfor the application of material to form a coating on at least onearticle, said article held on a carrier within a chamber in which thereis provided at least one magnetron in accordance with the invention asherein described.

In one embodiment the carrier is rotatable in said chamber.

In a further aspect of the invention there is provided a method ofoperating a magnetron to sputter deposit material from a target of saidmaterial provided with said magnetron, said method comprising the stepsof introducing a cooling fluid into the magnetron body, passing saidfluid to cool at least the target and a magnet array held within themagnetron body, such that said fluid passes substantially across all ofthe rear of the target, providing a power supply to the magnetron tocommence the sputter deposition of material, and wherein the powersupply level is increased for at least one phase of operation in whichthe current is increased at a rate greater than that of the voltagelevel.

In one embodiment the voltage level is held substantially constantduring said phase.

In one embodiment the magnetrons are operated in accordance with aclosed field unbalanced magnetron sputter ion plating method to depositmaterial onto said at least one article.

In one embodiment two magnetrons in accordance with the invention wereused to sputter carbon targets, to study sputtering characteristics,stability of plasma, and deposition rates. The targets were thenco-sputtered with a chromium target to produce the coatings in greatlyreduced deposition times.

In a further aspect of the invention there is provided an article havinga coating applied to at least one surface thereof, said coatingincluding material sputter deposited from at least one magnetron inaccordance with the invention.

In a further aspect of the invention, there is provided an article whichis to be used in a corrosive medium, said article having a coatingapplied to at least part of the surface thereof, said coating beingconductive and corrosion resistant and wherein the coating material issubstantially carbon based with a graphite microcrystalline structure.

In one embodiment the coating is of a type in which the carbon carbonbonding is mostly of the graphite sp2 form.

In one embodiment the coating has a specific wear rate under wetconditions of less than 10⁻¹⁶ m³/Nm.

In one embodiment the wear rate of the coating is 2.8×10⁻¹⁷ Nm

In one embodiment the coating is applied using a method as described inthe applicant's patent GB2331998.

In one embodiment, a layer of chromium or any other transition metal isapplied firstly to the article followed by the carbon material.

In one embodiment the coating formed is a mixture of carbon and chromiumor any other transition metal.

In one embodiment, the article to be coated is a plate used in a fuelcell.

In one embodiment the fuel cell is to be used in a vehicle.

Typically the coating which is formed is inert and conducting and cantherefore be used instead of precious metals such as gold and platinumin providing a conducting, corrosion resistant coating for applicationssuch as fuel cell plates.

In a further aspect of the invention there is provided a coatingcontaining Chromium doped carbon.

In one embodiment the coating is applied using a closed field unbalancedmagnetron sputter ion plating method which is found to show exceptionalwear resistance combined with high load bearing capability. In oneembodiment the coating is used in the production of thin, high qualitycarbon films for an application such as fuel cells.

Preferably the coating is applied using one or more magnetrons of thetype described in the first aspect of the invention. Typically themethod achieves the faster deposition rates required.

In one embodiment the coatings were produced at the faster depositionrates and specific wear rates of less than 5×10⁻¹⁷ m³/Nm obtained frompin on disc testing at 80N load (1 to 2 GPa).

In one embodiment the coating is a hydrogen-free amorphouscarbon-chromium coating, electrically conducting and containing mainlysp² bonding.

The apparatus and method in accordance with the invention is thereforeable to produce coatings at very high rates economically and this makesit possible to provide apparatus for applying coatings to articles whichare relatively large volume products, such as fuel cell plates.

It should therefore be appreciated that each of the above aspects of theinvention, independently, can provide an improved coating for specificarticles and/or improved. operation of the magnetrons used to sputterdeposit the material. It should also be appreciated that each of theaspects, in combination with one or more of the other aspects canprovide further advantages and indeed it is possible to incorporate allof the aspects into the magnetron.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention are now provided with reference tothe accompanying drawings, wherein:—

FIG. 1 illustrates a perspective view of a magnetron in accordance withone embodiment of the invention;

FIG. 2 illustrates a cross sectional elevation of the magnetron of FIG.1 along lines A-A, in accordance with a first embodiment of theinvention;

FIG. 3 illustrates a cross sectional elevation of the magnetron of FIG.1 along line B-B with a cooling liquid inlet and outlet located inaccordance with an embodiment of the invention;

FIG. 4 illustrates a graph showing the Voltage-Current characteristicsas the power applied to the magnetron is increased in accordance withthe invention;

FIGS. 5 and 6 illustrate two types of coating apparatus in accordancewith the invention.

FIG. 7 shows a schematic view of an experimental deposition chamber;

FIG. 8 shows current voltage characteristics of the magnetrons inaccordance with the invention (black symbols) and conventional (emptysquares) magnetrons with an Al target;

FIG. 9 shows the current-voltage characteristics of the magnetron inaccordance with the invention with a titanium target in a mixture ofargon and nitrogen;

FIG. 10 shows the current-voltage characteristics of the high power andthe conventional magnetrons with graphite target;

FIG. 11 shows the current to the substrates to be coated which arebiased by 60 V DC as a function of power of the magnetron with Ti and Altargets and the CM with Al target;

FIG. 12 shows characteristic OES spectra from the magnetron with Altarget at 0.17 Pa in argon, at discharge power 2.2 kW and 8.6 kW;

FIG. 13 shows current-voltage characteristics of magnetrons inaccordance with the invention;

FIG. 14 shows the surface quality of coatings showing very fewspots/defects (a) 9 A coating from the magnetron (b) 12 A coating fromthe magnetron.

FIG. 15 shows a taper cross-section of wear tracks, followingpin-on-disc tests at 80 N (Sliding distance 360 m, 8 mm diameter track,counterface:5 mm diameterWC/5 wt % Co ball)

(a) 3.5 A coating SWR=2.1×10⁻¹⁷ m³/Nm (b) 9 A coating SWR=2.8×10⁻¹⁷m³/Nm

(c) 12 A coating SWR=2.9×10−17 m³/Nm; and

FIG. 16 shows comparison of friction coefficients (80N Pin-on-Disc testagainst 5 mm diameter WC/6 wt % Co ball)

DETAILED DESCRIPTION OF THE INVENTION

Referring firstly to FIG. 1, there is provided a magnetron 2 having anouter body and supporting frame 4 with a front surface 6 which is formedat least partially by a front surface of the target 8 of material fromwhich particles 10 are sputtered in direction 12 to deposit onto asurface of an article (not shown).

To the rear and side of the target there is provided an array of magnetswhich will be described subsequently. The magnetron can be fitted andsealed within a coating chamber with the first face 6 facing into thechamber.

Referring to FIG. 2, there is illustrated the magnetron 2 in crosssection along line A-A in accordance with one embodiment of theinvention. The target of material 8 forms at least a portion of thefront surface 6 of the magnetron. The target in this case is formed ofcarbon from which particles can be deposited. The target is provided inthe supporting frame 4 which can be formed of metal and it may alsoinclude insulating material so as to separate the metal components fromthe cooling liquid and electricity supply 16 which passes to themagnetron to cause the operation of the same to sputter material.

To the rear of the magnetron, there is provided a series of magnets 18,said magnets provided in an array or configuration so as to suitspecific operating characteristics. The magnets are located, typicallyin an outer ring 18′ adjacent the periphery of the target and at leastone magnet 18 is provided to the centre of the target as shown. Inoperation, the magnets and/or target can heat up rapidly and thegeneration of heat can cause poor performance of the magnetron and/or aneed to reduce the power supply.

In an effort to reduce the heat which is generated, the power supply isconventionally limited to a lower level hence reducing the depositionrate of the sputtered material. A cooling liquid is also introduced intothe magnetron to pass through a channel 20 to provide a cooling effecton the target, frame and magnet.

Referring to the FIG. 2 embodiment, the channel depth 22 is limited bythe addition of filling material 24, said filling material typicallynon-magnetic and located a spaced distance from the rear face of thetarget so that the inner surface 26 forms a wall of the passage anddefines the depth 22 between the filling material 24 and the target 8 asshown. As the channel is restricted in depth, so the amount of liquidwhich is required to be introduced, is reduced and the flow of liquidbecomes turbulent which improves the cooling effect achieved.

FIG. 3 illustrates the magnetron of FIG. 1 in section along line B-B. Inthis case the inlet 30 and outlet 32 for the cooling liquid into andfrom the passage 20, are specifically located so as to lie at theopposing extremities of the magnetron. It is found that the positioningof the inlet and outlet 32 at the periphery of the magnetron, ensuresthat no air pockets are left in the channel 20 and that the liquid canflow across all of the area defined by the arrows 34, 36, therebyfurther improving the cooling effect which is achieved. Typically,channels (not shown) are provided which link the inlets and outlets tothe main cooling area with these channels typically provided behind themagnets.

As a result of the improved cooling which is provided, the power to themagnetron, and hence the deposition rate, can be increased.

Referring to FIG. 4, there is shown a graph which illustrates the mannerin which power can be applied to the magnetron in accordance with theinvention. In this case, it is shown in line 40 for a conventionalmagnetron that as the current increases so the voltage increases whichcauses damaged coatings to be created. However the graph lines 42, 44which represent the use of magnetrons in accordance with the inventionshow how in the initial phase of operation 43 from start the current andvoltage increase at a similar rate and thereafter, in the phase 41, thecurrent increases without the matching increase in the voltage, so it isthe current which is increased in particular thereby allowing theapplication of relatively high power levels to the magnetron, withrelatively low voltage values. The ability to kept the voltage levelsrelatively low avoids damage to the coatings while the increase incurrent and hence the power level allows increased coating speeds to beachieved. This allows increased power to be achieved in relation to themagnetron, hence increasing the deposition rate but without causing areduction in the quality of coating which is achieved.

The increase in the deposition rates which is possible has been found tobe more than 3 times that of the conventional apparatus and therebyallows the throughput of the coated articles to be increased and therebyrendering the overall process economical commercially, especially forthe coating of relatively high volume articles where previously electronbeam coating techniques would have been used.

Two examples of coating apparatus which can be used in accordance withthe invention are shown in FIGS. 5 and 6 respectively. In FIG. 5, thereis illustrated an in-line coating apparatus in which a series ofmagnetrons 52 are provided in pairs such that, each target in pair 52Ais of the same material and each target in pair 52B is of the samematerial. Thus the material which is deposited from the targets in anygiven pair, allow the application of a similar coating to opposing faces56 of the substrate 54 as they are passed through the coating chamber 60in the direction of arrows 62. The in-line coating apparatus allows therelatively high rate deposition of material onto the faces of thesubstrates.

FIG. 6 illustrates an alternative coating arrangement in plan. In thiscase the coating chamber 70 is provided with a series of magnetrons 72spaced around the same. Substrate carrier 64 is provided to rotate asindicated by arrow 66 so as to allow substrates (not shown) mounted onthe external surface 68 of the carrier 64, to be rotated past each ofthe magnetrons 72 which can be selectively operated to sputter materialfrom the targets mounted thereon at any given time. In one embodiment,the magnetrons may be provided as part of a closed field unbalancedmagnetron sputter ion plating apparatus.

A schematic representation of an experimental apparatus for themagnetron testing is shown on FIG. 7. The vacuum chamber has a diameterof 650 mm and height 600 mm. Prior to experiments it was pumped by aLeybold T-1600 turbopump typically to 6.7·10⁻⁶ Pa base pressure. Gaspressure in the chamber was set by regulating gas flow by MKS mass-flowcontrollers. In these experiments argon flow was regulated between 7.5and 75 sccm. When the magnetron was operated with a Ti target nitrogenflow was set at approximately 0.6 that of the argon flow to verifyoperation of the magnetron (HPM) during the reactive process.

Two unbalanced high power magnetrons (HPM) in accordance with theinvention were used with standard industrial size rectangular targets380 mm by 175 mm were mounted opposite each other in the chamber. Theyformed a closed field magnetic configuration. The magnetrons differ fromthe conventional magnetron (CM) by a stronger magnetic field above thetarget and optimised water cooling system. They were powered by AdvancedEnergy MDX-II DC generators, capable of producing up to 15 kW outputpower. When current-voltage characteristics were recorded, one HPM wasoperated at a time with the generator run in current regulation mode.

The optical emission spectra (OES) were recorded through a viewport atthe top lid of the chamber using EP200 Verity Instruments monochromator.The line of sight of the viewport, covered by the fused quartz window,was passing at 20 mm from the magnetron target above the longer side ofits sputter track.

During deposition of the TiN coating, nitrogen supply in the chamber wasregulated by a piezo valve, driven by a controller linked with opticalemission monitor, which was set at 497 nm emission line of titanium.Coating properties were studied using a Fischerscope H100 hardnesstester and TCL ST-3001 Tribo tester in the mode of unidirectionalscratch.

The current-voltage characteristics of the high-power magnetron withaluminium target are presented in FIG. 8. Pressure change from 0.09 to0.7 Pa did not affect significantly the shape of the curves. With theincrease of the discharge current the cathode voltage saturates startingfrom 16 A, staying practically flat up to the maximum output power ofthe DC generator. At 0.17 Pa the cathode voltage had a weak maximum 278V with its value dropping by 5 V above 30 A current on the target. Onthe contrary the cathode voltage of the standard magnetron did notsaturate with current increase, significantly exceeding the cathodevoltage of the HPM run at the same pressure of 0.3 Pa.

The current-voltage characteristics of an HPM with a titanium target arepresented in FIG. 9. Saturation of the cathode voltage occurred ataround 4 to 6 A, then the voltage rises slightly as the current isincreased. Small humps can also be seen in both plots. This complexnature of the curves is linked with the formation of a nitride layer onthe target surface, which is sputtered away at the higher values ofcurrent. Examination after opening the deposition chamber revealed asurface of the erosion track having a silvery metallic colour surroundedby a characteristic yellow deposit on the unsputtered parts of thetarget.

Current-voltage characteristics of the HPM and Conventional magnetron(CM) with graphite targets are presented on FIG. 10. They demonstratethe same tendency as during the runs with the aluminium target. The HPMcurve displays a much slower rise of voltage with current increase incomparison with the conventional magnetron (CM) and allows operation ofgraphite target at higher power levels without arcing occurring.

Operation of the HPM at powers up to 15 kW draws significantly highercurrent to the samples as shown in FIG. 11. Current to the samples fromHPM plasmas increases starting from 1.5 kW on the target, while in thecase of conventional magnetron it increases straightaway. It may beattributed to a better confinement of plasma above the HPM target at lowdischarge power due to its stronger magnetic field. Further, almostlinear growth of the current occurs for both magnetrons with the currentfrom HPM plasma 2.5 times exceeding the one from the conventionalmagnetron.

OES spectrum were recorded during operation of the HPM with an aluminiumtarget at 2.2 kW power characteristic to deposition processes withconventional magnetron, and 8.6 kW and are shown in FIG. 12. At the lowpower the most prominent peaks are with their maximum at 308 and 395 nm,which are associated with emission lines of AlI atoms 308.2, 309.3,394.4 and 396.2 nm. At the high power they continue to dominate thespectrum with growth of the AlI lines at 257.5 and 266 nm. Also ArIlines at 415.9, 425.9, 430 and 433.4 nm become quite noticeable.Appearance of the emission lines at 281.5 and 358.7 nm, corresponding tothe transitions of AlII ions, indicates partial ionisation of thesputtered material. But their small value in comparison with the linesof neutral aluminium points out that conventional sputtering by argonions dominates the process. Substantial input of self-sputtering modecould be expected at power density around 300 W/cm² on the target.

Trial deposition run of TiN coating was conducted using two unbalancedHPMs (FIG. 7) powered by 12 kW each. M42 tool steel samples were mountedon the ring sample holder (FIG. 7), which rotated at 4 rev/min.Separation from the magnetron targets, when samples were passing infront of it, was 15 cm.

Resulting coating had hardness 33 GPa and good adhesion properties. Nodelamination of the coating was observed in the scratch track up to 60 Nmaximum load. No droplets could be detected by SEM on the coatingsurface. Total thickness, measured by ball crater method, was 3.36 μm,which gave deposition rate 4 μm/h. For comparison, in a conventionalmagnetron system at 6 kW net power TiN coating is deposited at 1-1.2μm/h. These results demonstrate that deposition rates could be increasedin comparison with conventional magnetron processes proportionally tothe power applied to the sputter target. Also, rates of coating from HPMapproach those obtained from arc evaporation technique.

The magnetrons in accordance with the invention in another example areoperated at power up to 15 kW with power density on the target up to 40W/cm², which is more than 3.5 times that of the conventional magnetron.The current-voltage characteristics indicate that much higher powercould be used. The deposition rate of the magnetron rival that of arcsources but without the associated droplet formation.

In a further set of trials relating to coatings formed using themagnetron (HPM), the following was performed. Coatings were deposited ina Teer UDP650/4 closed field unbalanced magnetron sputtering system withfour magnetrons. For all coatings, the deposition chamber was configuredwith two carbon targets (opposite one another) and two chromium targets.Each UDP650 target was 345×145 mm, mounted on a 380×175 mm copperbacking plate. Initially, the sputtering system was used withconventional magnetrons, to deposit coatings at 3.5 A current input tothe carbon targets, (i.e. at maximum power input possible for stableoperation: 2.4 kW, 36 kWm⁻²). DC power was used on the targets andsubstrates were biased with a pulsed DC supply. A typical coatingsequence was used, i.e. ion cleaning, deposition of a Cr adhesion layer,then deposition of a Cr/C graded layer to change composition from purechromium to the composition of the tribological coating (carboncontaining ^(˜)7 at % Cr). Parameters are then kept constant until theend of the process. Test pieces coated were M42 high speed steel, groundto a 1200 SiC finish and cleaned ultrasonically in acetone beforecoating.

For the high deposition rate coatings, two HPMs were used to replace theconventional magnetrons supporting the carbon targets. The HPM designincluded redesigned water cooling cavities to promote more efficienttarget cooling, and a stronger magnetic arrangement than that usedpreviously. The magnetrons used for the chromium targets were notchanged. Prior to loading any samples, the HPM magnetrons were operatedat different powers, and the current—voltage characteristics recorded.It was found possible to sputter the carbon targets, withoutinstability, at currents up to 12 A (560 to 590V) and target powers upto ^(˜)7 kW (104 kWm⁻²) compared to the 2.4 kW (36 kWm⁻²) achievedpreviously.

Substrates were loaded into the chamber, and coated with pure carbon,using high target currents (9 A to 12 A) to establish carbon depositionrates. Required increases in chromium target power were then estimatedbased on the carbon deposition rates achieved. Coating trials were thencarried out at the higher carbon and chromium target current values,using the deposition procedure described above, although depositiontimes for the final layer were reduced to achieve a coating of therequired thickness. Tribological tests (detailed below) were used toassess coating performance. Further coatings were then produced atdifferent carbon target current values from 2.5 A (1.1 kW) to 12 A (7kW), to assess the suitability of the HPMs for use over a range oftarget powers. The ion current at the sample fixture was monitoredduring all coating process runs.

Optical microscopy was used to examine the surface of the coatings, andto study wear tracks following tribological testing. Coating thicknesseswere assessed using the ball crater taper-section technique. A standardhardness tester (Wilson/Rockwell B503-R) using a load of 150 kgf wasused to assess the adhesion of the coatings. The plastic microhardnesswas determined using a Fischerscope H100 ultramicro-hardness tester witha Vickers indenter, from the load penetration curves. Data from fiveindents made on each sample were averaged. The loading/unloading ratewas 10 mNs⁻¹, with a maximum applied load of 50 mN.

A Teer POD-2 pin-on-disc tester was used to assess the tribologicalperformance of the coatings against a 5 mm diameter WC/6% Co ball under80N applied normal load. Tests were performed at 200 mms⁻¹ linear speedon an 8 mm diameter wear track, for a sliding distance of 360 m. Alltests were unlubricated and at room temperature (˜25° C.) and relativehumidity (˜35%). The friction coefficient was monitored using astrain-gauge load cell, and the wear volume measured by producing a ballcrater taper-section on the wear track. The wear volume was thennormalised with respect to the load and sliding speed to give specificwear rates.

Coating structures were analysed by X-ray Diffraction using a Philips PW1070/30 instrument and θ/2θ (Bragg-Brentano) configuration. Cu Kαradiation source, 40 kV and 35 mA on the target, was used for themeasurement. Scan conditions were: 2θ range of 20-100°; step scan:0.02°/step, 0.4 sec./step; a Graphite monochromater was used; slitcombination 1°-0.5°-0.5-1. The approximate chemical compositions ofsamples were obtained using Glow Discharge Spectroscopy (GDS) in a LECOGDS-750 QDP apparatus.

FIG. 13 shows the current-voltage curves obtained for each of the highpower magnetrons tested, compared with the curve for a conventionalmagnetron. For the new design of magnetron, large increases in targetcurrent are obtained with only small increases in target voltage,allowing it to be operated at much higher target powers withoutintroducing instabilities such as arcing. Ion current values recorded onthe test piece sample fixture during deposition of the high powercoatings were significantly higher (×2.5 to ×3.5) than for the 3.5 Acoating as would be expected. This is due to the high flux of ions andelectrons from the unbalanced magnetron, which increases as the powerinput is increased. The sample fixture was the same for all three testshence it can be assumed that the ion current density has increasedsignificantly. Although, temperatures at the substrate fixture were notmonitored during these trials, it can be assumed that these higher ioncurrent densities would have resulted in higher substrate temperatures,and this would need to be considered when assessing the suitability ofthe high power process to coat temperature sensitive materials.Operation of the HPM at lower powers resulted in similar ion currentvalues to those normally seen for the conventional magnetron at thosepowers.

Table I shows the coating thickness measurements and relative depositionrates for the standard coating, and two of coatings produced at muchhigher power with the new magnetron design.

TABLE I Comparative coating deposition rates. Coating thickness CarbonCr Deposition Deposition target inter- Time rate Magnetron current/layer/ GraphitiC/ Total/ GraphitiC/ GraphitiC/ Design A μm μm μm hrsμm/hr Con- 3.5 0.2 1.8-2.0 2.0-2.2 4 0.45-0.5  ventional HPM 9.0 0.21.8-1.9 2.0-2.1 2 0.9-0.95 HPM 12.0 0.2 2.4/2.5 2.6/2.7 2 1.2/1.35

It can be seen that the deposition rate has been increased by up to 2.7times depending on the operating conditions chosen. This enables thedeposition time to be more than halved. These times are for theparticular size of target used in the UDP650 system which is arelatively small production system or large R&D system. Deposition ratesin larger production systems, often with six larger targets, are twicethat of the smaller systems, and should the same designs be applied tothe magnetrons in these, deposition rates could again be increased.

All coatings produced demonstrated good adhesion to the M42 substrate:Rockwell indentations produced in the coatings were classed as HF1 toHF2. FIGS. 14 a-b shows the surface quality of the HPM coatings. The 9 Acoatings (FIG. 14 a) showed very little evidence of spots or defects andgenerally appeared better than typically seen for 3.5 A coatingsproduced with conventional magnetrons. The surface quality of the 12 Acoating (FIG. 14 b) appeared comparable to that usually seen for a 3.5 Acoating. i.e. a very low level of surface defects were visible. Theplastic hardness values were 1519 to 1729 kgmm⁻² for the 9 A coatings,and 1554 kgmm⁻² for the 12 A coating. This compares to 1769 kgmm⁻²obtained for the 3.5 A coating with Hardness values typically rangingfrom 1500 kgmm⁻² to 1700 kgmm⁻² in this coating system.

Pin-on-disc tests as indicated in FIGS. 15 a-c, under high (80N) loadconfirmed that the coatings had similar characteristics to thoseproduced at lower power, as shown in FIG. 16, demonstrating the samehigh load bearing capability and low friction characteristics. Wearrates measured on taper cross-sections produced by ball cratering on thewear tracks were <3×10⁻¹⁷ m³/Nm and friction coefficients were 0.07/0.08for 9 A coatings and 0.09 for the 12 A coating. This data and theresults from the hardness tests, suggest that coatings produced inreduced deposition times should offer the same wear resistance tocomponents operating in air as currently achievable with typicalcoatings.

Table II shows that the HPMs can also be operated at lower target powersand currents if coatings are required on more temperature sensitivesubstrates.

TABLE II Wear properties of coatings produced with the HPM at differentcurrents. Specific C Wear Rate at Final Magnetron Target Cr Target 80NPOD/ Friction Design Current*/A Current**/A m³N⁻¹m⁻¹ CoefficientConventional 3.5 0.25 2.1 × 10⁻¹⁷ 0.07 HPM 2.5 0.18 2.7 × 10⁻¹⁷ 0.07 HPM5.0 0.25 3.3 × 10⁻¹⁷ 0.08 HPM 9.0 0.40 2.8 × 10⁻¹⁷ 0.08 HPM 12.0 0.542.9 × 10⁻¹⁷ 0.09 *applied to each of two carbon targets *applied to oneof two chromium targets

X-ray diffraction of 9 A and 12 A coatings produced traces identical tothat for the 3.5 A coating, showing only peaks characteristic ofuncoated M42 high speed steel substrates. No other peaks could beidentified suggesting that all three coatings had the same amorphousstructure characteristic of the conventional formed coatings.

Analysis by GDS showed the chromium content to fall from approximately 6at % for the 3.5 A coating to around 3 at % and 1 at % for the 9 A and12 A coatings respectively, suggesting that the Cr content was lowerwithin the coatings sputtered at faster rates. The chromium targetpowers chosen were based on the pure carbon deposition rates obtained,and assumption that the chromium sputter rate would increase linearlywith power input to the target. However, factors such ascross-contamination of targets and increased ionization within thechamber may have also influenced the chromium sputter rate. The lowerchromium content was not found to be detrimental to the toughness andload bearing capability of the coating, but future work shouldinvestigate variations to Cr content.

Coatings were produced by sputtering carbon targets from a new magnetrondesign, with stronger magnetic field and enhanced target cooling.Operation of the high power magnetrons was possible at approximatelythree times the power input achievable with conventional designs, andthis enabled sputter rates to be increased by up to 2.7 times anddeposition times to be significantly reduced. Tribological and physicalproperties of the coatings deposited were not found to be greatlyinfluenced by the fast deposition rates and coatings produceddemonstrated good wear rates of <5×10⁻¹⁷ m³/Nm when tested inatmospheric conditions against WC/6 wt % Co counterfaces at high (80N)load.

Properties of the coatings such as surface quality and adhesion of thecoatings which are achieved using the apparatus and method of theinvention are found to be good hence allowing the application ofcoatings onto articles such as plates for fuel cells to be achievedusing sputtering rather than electron beam techniques. The reduction inprocess times achievable with the use of the magnetrons in accordancewith the invention is an important factor in the drive to advanceprocess efficiency, economic aspects and suitability for a wider rangeof applications.

The provision of coatings of the graphitic type as herein described ontoarticles which require conductivity and wear resistance but without theneed for high priced inert metals to be used also represents asignificant development in the manufacture of this type of article.

The invention claimed is:
 1. An article, said article comprising: acoating applied to at least part of the surface thereof, said coatingbeing conductive and corrosion resistant, the coating comprises a layerincluding a transition metal applied to a surface of the article and amaterial applied thereto which is substantially carbon based with agraphite microcrystalline structure; said article is electricallyconductive and the said coating is applied using apparatus including achamber in which the article is held with an electrical bias on acarrier, and at least one magnetron with a target of the material whichis sputter deposited onto the article such as to allow the article andcoating to continue to be electrically conductive when placed in acorrosive medium with the coating acting to protect the article from thecorrosive medium and a power supply is provided to the at least onemagnetron to cause the sputter deposition of material therefrom andwherein the level of the power supply is increased during the sputterapplication of the material to form the coating on the article and thereis at least a phase during the sputter application in which the currentof the power supply is increased at a greater rate than the voltage ofthe power supply.
 2. The article according to claim 1 wherein saidcoating is produced at specific wear rates of less than <5×10⁻¹⁷ m³/Nmfrom pin on disc testing at 18 newtons load.
 3. The article according toclaim 1 wherein said coating is a hydrogen free amorphous carbonchromium coating which is electrically conducting and contains mainlysp2 carbon carbon bonds.
 4. The article according to claim 1 whereinsaid coating is of a type in which the carbon carbon bonding issubstantially of the graphitic sp2 form.
 5. The article according toclaim 1 wherein said coating has a specific wear rate under wetconditions of less than 10⁻¹⁶ m³/Nm.
 6. The article according to claim 1wherein said coating has a wear rate of 2.8×10⁻¹⁷ Nm.
 7. The articleaccording to claim 1 wherein said coating formed on the transition metallayer is a mixture of carbon and a transition metal.
 8. The articleaccording to claim 1 wherein the article is a plate used in a fuel cell.9. The article according to claim 8 wherein said fuel cell is to be usedin a vehicle.
 10. Apparatus for forming a coating on an article asdescribed in claim
 1. 11. Apparatus according to claim 10 wherein saidmagnetron includes a magnet array comprising a series of spaced magnetslocated to the rear of the target.
 12. Apparatus according to claim 11wherein the magnetron includes an inlet located at or adjacent to afirst extremity or edge of the magnetron and an outlet located at oradjacent to a further edge or extremity of the magnetron to allow acooling liquid to enter the magnetron at the inlet, pass acrosssubstantially all of the area of the rear face of the target of themagnetron and across the magnets in the magnet array and leave via theoutlet and wherein the outlet and inlet lie to the rear of said magnetarray and in line with the magnet or magnets mounted at the periphery ofthe magnetron.
 13. A magnetron according to claim 10 wherein the saidphase occurs after an initial phase at the commencement of operation ofthe magnetron, during which initial phase both voltage and currentlevels of the power supply increase and then during said phase thevoltage level is substantially constant while the current levelincreases.
 14. A method of forming a coating on an article as set forthin claim 1, said method comprising the steps of: placing the articleinto a chamber in which the article is held with an electrical bias on acarrier; providing a power supply to operate at least one magnetron witha target of the material to be used to form the coating; operating themagnetron to sputter deposit the material onto the article; andcontrolling the level of the power supply to increase the same duringthe sputter application of the material to form the coating on thearticle and wherein for at least a phase of providing the power supplythe current of the power supply is increased at a greater rate than thevoltage of the power supply.